1. Field of the Invention
The present invention relates to a method of and an apparatus for designing an EB (Electron Beam) mask, which is used in an electron beam exposing apparatus to draw a predetermined pattern on a semiconductor substrate.
2. Description of the Related Art
In recent years, in a process for manufacturing a semiconductor integrated circuit apparatus, a micro fabrication technique has been actually used for using a focused beam of a charged particle line, such as an electron beam, an ion beam or the like, and then drawing an integrated circuit pattern. For example, an electron beam exposing apparatus radiates an electron beam onto a wafer on which electron line sensitive resist is coated, and accordingly exposing an integrated circuit pattern. At this time, an EB mask is used in order to obtain a drawing pattern through the electron beam.
As an electron beam usage drawing technique using an electron beam, a partially collectedly or entirely collectedly exposing technique is known for contracting and projecting a pattern of the EB mask, and then collectedly drawing a unit region, such as a memory cell and the like, on a wafer.
Those exposing methods usually use two masks. At first, a first mask is used to adjust the electron beam to a rectangular shape. Next, the adjusted electron beam is radiated to a second mask. The second mask has a plurality of rectangular cell apertures where a partial pattern in which a part of an integrated circuit pattern to be radiated onto the wafer is picked up is formed. Then, these cell apertures are reduced to several one-tenths by an electronic optical system, and transcribed onto the wafer. Accordingly, the collected exposure is done. Such a partially collectedly or entirely collectedly exposing method not only reduces the number of shots to improve throughput, but also improves the connection accuracy of the shots, the picture quality of a slant pattern and the compressive performance of pattern data. Thus, this has an excellent point that even if the hyperfineness is increased, it does not have a direct influence on a wafer drawing time.
Incidentally, in the mask used in the collectedly exposing method as mentioned above, there are a stencil type EB mask in which a mask hole for allowing the electron beam to pass through is made correspondingly to the integrated circuit pattern, and a membrane type EB mask in which a film for shielding the electron beam is formed correspondingly to the integrated circuit pattern.
As shown in FIG. 1A, the stencil type EB mask in the two kinds has a problem that a region whose circumference is perfectly surrounded with the mask hole (slant line portion) can not be prepared since there is no portion for supporting it (hereafter, referred to a donut problem). As shown in FIG. 1B, in a region whose circumference except a slight portion is surrounded with the mask hole (slant line portion), it is impossible to sufficiently have the strength of the portion to support it. Thus, this brings about a problem that the supporting portion is deformed and damaged (hereafter, referred to as a leaf problem).
So, in order to prepare the stencil type EB mask, the donut problem and the leaf problem are solved as follows. That is, the conventional method of designing the EB mask divides the integrated circuit pattern into two complementary patterns, and makes the mask holes in the two EB masks, respectively. For example, it is prepared as shown in FIG. 2A, for the integrated circuit pattern shown in FIG. 1A, and it is prepared as shown in FIG. 2B, for the integrated circuit pattern shown in FIG. 1B. Then, the integrated circuit pattern is contracted and transcribed onto the wafer by the sequential exposure using the two EB masks on which the complementary masks are formed (hereafter, it may be referred to as a complementary mask).
When the complementary pattern is formed on the stencil type EB mask, a photolithography technique is usually used to form a desirable resist pattern on a mask, and a mask hole is made by etching. Here, when the resist pattern is formed on the mask, an optimal exposure amount is different between a pattern having an extremely wide line width and a pattern having a narrow line width. Thus, it is desirable to avoid an open (mask hole) pattern of a large area as much as possible.
Also, when the integrated circuit pattern is divided into two complementary patterns, in order to make the optimal exposure amount constant and reduce the blur of the electron beam caused by a coulomb effect at the time of the contraction and the transcription onto the wafer, it is desirable to make the area densities of the mask holes of the two complementary masks equal to each other.
For this reason, the conventional method of designing the EB mask not only considers the donut problem and the leaf problem, but also tries to remove the existence of the mask hole having the large area. At the same time, it makes the area densities of the mask holes of the two complementary masks equal to each other. For this reason, it employs a method of cutting and dividing the integrated circuit pattern at a predetermined length.
However, in such a dividing method, when the pattern of the large area exists in the integrated circuit pattern, if it is divided into two complementary patterns so that the area densities are equal to each other, there may be a case that a point contact pattern at which the mask holes are linked at one point is generated, as shown in FIG. 3. Also, there may be a fear that a micro bridge pattern at which the mask holes are linked in a micro dimensional pattern is generated, as shown in FIG. 4.
The point contact pattern and the micro bridge pattern are weak in mechanical strength. Thus, there may be a fear that the EB mask is damaged. For this reason, such a complementary mask can not be prepared (hereafter, referred to as a checkered flag problem).
The conventional apparatus for designing the EB mask does not have the function of detecting the point contact pattern or the micro bridge pattern. For this reason, a designer detects the point contact pattern or the micro bridge pattern in a visual manner or the like. Then, if there is the point contact pattern or the micro bridge pattern, the shape of the complementary pattern is changed to then cope with the checkered flag problem.
However, if the person detects the point contact pattern or the micro bridge pattern and modifies the complementary pattern as mentioned above, this requires a large number of design steps and an expensive cost, and TAT becomes long. Also, the oversight of the point contact pattern or the micro bridge pattern disables the occurrence of the checkered flag problem to be perfectly solved.
Japanese Laid Open Patent Application (JP-A-Heisei, 11-237728) discloses the following drawing method and drawing apparatus.
The first drawing method includes the steps of dividing a drawing pattern including repetition patterns into a frame in which a boundary line is inclined for an array direction of the repetition patterns, and a step of drawing by using a charged particle beam or a laser beam for each frame with regard to a sample.
The second drawing method includes the steps of dividing a drawing pattern including repetition patterns into a frame in which a boundary line is inclined for an array direction of the repetition patterns, and a step of setting the boundary line of the frame at a serration state and then drawing by using a charged particle beam or a laser beam for each frame with regard to a sample.
Its drawing apparatus is the drawing apparatus having the device that can carry out the two steps of the first drawing method.
Japanese Laid Open Patent Application (JP-A-Heisei, 10-284394) discloses the following method of and apparatus for exposing a charged beam.
This method of exposing the charged beam installs a mask in a beam path of an optical mirror cylinder for a charged beam, and divides at least a part of a pattern on the mask into small regions, and places on the mask, and then changes an illumination beam to the small regions on the upstream side from the mask, and thereby illuminates to a particularly small region, and creates and transcribes an image of the small region onto an exposed surface, and further couples and arrays the small regions on the exposed surface, and accordingly generates at least a part of a predetermined large region pattern, and this method of exposing the charged beam is characterized in that it detects an alignment mark placed on the exposed surface, and thereby detects a positioning error in a rotational direction on the exposed surface, and then uses a rotation lens mounted in a projection lens system between the mask and the exposed surface, and then rotates the transcription image of the mask pattern, and thereby compensates the error, and accordingly improves the linkage accuracy between the transcribed images adjacent to each other on the exposed surface.
Also, the apparatus for exposing the charged beam is the apparatus for exposing the charged beam, which has a unit that can carry out the above-mentioned method of exposing the charged beam.
Japanese Laid Open Patent Application (JP-A-Heisei, 9-129544) discloses the following method of transcribing a charged particle line.
In this method of transcribing a charged particular line which divides a transcription pattern formed on a mask into a plurality of main visual fields, and also divides each of the plurality of main visual fields into a plurality of sub visual fields, and sequentially radiates a charged particle line for each of the plurality of sub visual fields, and then transcribes the pattern of each of the sub visual fields through an image creation lens system onto a substrate, and also shifts the mask and the substrate in synchronization with each other, and accordingly transcribes the transcription pattern onto the substrate, this method of transcribing the charged particle line is characterized in that it compensates the distortion of the pattern transcribed onto the substrate for each sub visual field.
The present invention is accomplished in view of the above mentioned problems.
Therefore, an object of the present invention is to provide a method of and an apparatus for designing an EB mask which can surely detect a point contact pattern and a micro bridge pattern, and cope with a checkered flag problem.
Another object of the present invention is to provide a method of and an apparatus for designing a stencil type EB mask in which not only the donut problem and the leaf problem but also the checkered flag problem can be solved.
Still another object of the present invention is to provide a method of and an apparatus for designing an EB mask in which it is not necessary for the designer to detect the point contact pattern and the micro bridge pattern and modify the complementary pattern.
In order to achieve an aspect of the present invention, the present invention provides a method of designing EB (electron beam) mask, including the steps of (a) to (d). The step of (a) is the step of dividing an integrated circuit pattern into two complementary patterns. The step of (b) is the step of scanning around one, as an object pattern, of small patterns included in one of the two complementary patterns, while measuring a distance from the object pattern to the small patterns adjacent to the object pattern. The step of (c) is the step of registering a minimum of distances from the object pattern to the adjacent small patterns. The step of (d) is the step of changing a shape of at least one of the small patterns based on the minimum distance.
In the method of designing EB mask, the step of (d) may include the step of (e). The step of (e) is the step of adding an auxiliary pattern between the object pattern and the most adjacent one of the adjacent small patterns.
In the method of designing EB mask, the step of (e) may includes the steps of (f) and (g). The step of (f) is the step of adding the auxiliary pattern in the one complementary pattern. The step of (g) is the step of deleting a pattern having the same shape as that of the auxiliary pattern from a place corresponding to the auxiliary pattern in the other of the two complementary patterns.
In the method of designing EB mask, the step of (d) may includes the step of (h). The step of (h) is the step of shifting a part of the one complementary pattern in a predetermined direction.
In the method of designing EB mask, the step of (h) may includes the steps of (i) and (j). The step of (i) is the step of shifting the part of the one complementary pattern in the predetermined direction. The step of (j) is the step of deleting a pattern having the same shape as that of the part from a place corresponding to the part in the other of the two complementary patterns.
In the method of designing EB mask, the step of (b) may includes the step of (k). The step of (k) is the step of stopping the scan, if the distance exceeds a threshold.
In order to achieve another aspect of the present invention, the present invention provides an apparatus for designing EB mask including a memory and a processor. The memory stores data. The processor divides an integrated circuit pattern into two complementary patterns; scans around one, as an object pattern, of small patterns included in one of the two complementary patterns, while measures a distance from the object pattern to the small patterns adjacent to the object pattern; registers a minimum of distances from the object pattern to the adjacent small patterns in the memory; and changes a shape of at least one of the small patterns based on the minimum distance.
In the apparatus for designing EB mask, the processor may add an auxiliary pattern between the object pattern and the most adjacent one of the adjacent small patterns.
In the apparatus for designing EB mask, the processor may add the auxiliary pattern in the one complementary pattern; and delete a pattern having the same shape as that of the auxiliary pattern from a place corresponding to the auxiliary pattern in the other of the two complementary patterns.
In the apparatus for designing EB mask, the processor may shift a part of the one complementary pattern in a predetermined direction.
In the apparatus for designing EB mask, the processor may shift the part of the one complementary pattern in the predetermined direction; and delete an pattern having the same shape as that of the part from a place corresponding to the part in the other of the two complementary patterns.
In the apparatus for designing EB mask, the processor may stop the scan, if the distance exceeds a threshold.